/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.catalina.startup;

import org.apache.catalina.Context;
import org.apache.tomcat.util.buf.UriUtil;

import javax.servlet.ServletContext;
import java.io.*;
import java.net.URL;
import java.nio.charset.Charset;
import java.util.*;
import java.util.regex.Pattern;

/**
 * A variation of Java's JAR ServiceLoader that respects exclusion rules for
 * web applications.
 * <p/>
 * Primarily intended for use loading ServletContainerInitializers as defined
 * by Servlet 8.2.4. This implementation does not attempt lazy loading as the
 * container is required to introspect all implementations discovered.
 * <p/>
 * If the ServletContext defines ORDERED_LIBS, then only JARs in WEB-INF/lib
 * that are named in that set will be included in the search for
 * provider configuration files; if ORDERED_LIBS is not defined then
 * all JARs will be searched for provider configuration files. Providers
 * defined by resources in the parent ClassLoader will always be returned.
 * <p/>
 * Provider classes will be loaded using the context's ClassLoader.
 *
 * @see javax.servlet.ServletContainerInitializer
 * @see java.util.ServiceLoader
 */
public class WebappServiceLoader<T> {
	private static final String LIB = "/WEB-INF/lib/";
	private static final String SERVICES = "META-INF/services/";
	private static final Charset UTF8 = Charset.forName("UTF-8");

	private final Context context;
	private final ServletContext servletContext;
	private final Pattern containerSciFilterPattern;

	/**
	 * Construct a loader to load services from a ServletContext.
	 *
	 * @param context the context to use
	 */
	public WebappServiceLoader(Context context) {
		this.context = context;
		this.servletContext = context.getServletContext();
		String containerSciFilter = context.getContainerSciFilter();
		if (containerSciFilter != null && containerSciFilter.length() > 0) {
			containerSciFilterPattern = Pattern.compile(containerSciFilter);
		} else {
			containerSciFilterPattern = null;
		}
	}

	/**
	 * Load the providers for a service type.
	 *
	 * @param serviceType the type of service to load
	 * @return an unmodifiable collection of service providers
	 * @throws IOException if there was a problem loading any service
	 */
	public List<T> load(Class<T> serviceType) throws IOException {
		String configFile = SERVICES + serviceType.getName();

		LinkedHashSet<String> applicationServicesFound = new LinkedHashSet<String>();
		LinkedHashSet<String> containerServicesFound = new LinkedHashSet<String>();

		ClassLoader loader = servletContext.getClassLoader();

		// if the ServletContext has ORDERED_LIBS, then use that to specify the
		// set of JARs from WEB-INF/lib that should be used for loading services
		@SuppressWarnings("unchecked")
		List<String> orderedLibs =
				(List<String>) servletContext.getAttribute(ServletContext.ORDERED_LIBS);
		if (orderedLibs != null) {
			// handle ordered libs directly, ...
			for (String lib : orderedLibs) {
				URL jarUrl = servletContext.getResource(LIB + lib);
				if (jarUrl == null) {
					// should not happen, just ignore
					continue;
				}

				String base = jarUrl.toExternalForm();
				URL url;
				if (base.endsWith("/")) {
					url = new URL(base + configFile);
				} else {
					url = UriUtil.buildJarUrl(base, configFile);
				}
				try {
					parseConfigFile(applicationServicesFound, url);
				} catch (FileNotFoundException e) {
					// no provider file found, this is OK
				}
			}

			// and the parent ClassLoader for all others
			loader = context.getParentClassLoader();
		}

		Enumeration<URL> resources;
		if (loader == null) {
			resources = ClassLoader.getSystemResources(configFile);
		} else {
			resources = loader.getResources(configFile);
		}
		while (resources.hasMoreElements()) {
			parseConfigFile(containerServicesFound, resources.nextElement());
		}

		// Filter the discovered container SCIs if required
		if (containerSciFilterPattern != null) {
			Iterator<String> iter = containerServicesFound.iterator();
			while (iter.hasNext()) {
				if (containerSciFilterPattern.matcher(iter.next()).find()) {
					iter.remove();
				}
			}
		}

		// Add the application services after the container services to ensure
		// that the container services are loaded first
		containerServicesFound.addAll(applicationServicesFound);

		// load the discovered services
		if (containerServicesFound.isEmpty()) {
			return Collections.emptyList();
		}
		return loadServices(serviceType, containerServicesFound);
	}

	private void parseConfigFile(LinkedHashSet<String> servicesFound, URL url)
			throws IOException {
		InputStream is = null;
		BufferedReader reader = null;
		try {
			is = url.openStream();
			InputStreamReader in = new InputStreamReader(is, UTF8);
			reader = new BufferedReader(in);
			String line;
			while ((line = reader.readLine()) != null) {
				int i = line.indexOf('#');
				if (i >= 0) {
					line = line.substring(0, i);
				}
				line = line.trim();
				if (line.length() == 0) {
					continue;
				}
				servicesFound.add(line);
			}
		} finally {
			if (reader != null) {
				reader.close();
			}
			if (is != null) {
				is.close();
			}
		}
	}

	private List<T> loadServices(Class<T> serviceType, LinkedHashSet<String> servicesFound)
			throws IOException {
		ClassLoader loader = servletContext.getClassLoader();
		List<T> services = new ArrayList<T>(servicesFound.size());
		for (String serviceClass : servicesFound) {
			try {
				Class<?> clazz = Class.forName(serviceClass, true, loader);
				services.add(serviceType.cast(clazz.newInstance()));
			} catch (ClassNotFoundException e) {
				throw new IOException(e);
			} catch (InstantiationException e) {
				throw new IOException(e);
			} catch (IllegalAccessException e) {
				throw new IOException(e);
			} catch (ClassCastException e) {
				throw new IOException(e);
			}
		}
		return Collections.unmodifiableList(services);
	}
}
